Polyester resins are used in molding materials, films, adhesives, coating materials, toner binder resins, binder resins for sublimation transfer films, ink binders, modifiers, and the like.
For instance, when polyester resins are used as coating materials and binder resins, it is necessary for the polyester resins to be soluble in the solvent.
In addition, toners need to maintain stable conditions during storage without resulting in agglomeration, and to provide high-resolution images. To provide high-resolution images, toner binder resins need to be pulverized into fine particles.
Moreover, to achieve high-resolution images, chemical methods capable of forming fine particulate toners are drawing attention.
An example of such chemical methods to obtain toner particles is as follows:
a polyester resin and toner additives such as a colorant are dissolved or dispersed in an organic solvent capable of dissolving the polyester resin; the mixture liquid is dispersed in an aqueous medium containing a dispersion stabilizer or the like so that particles are formed; the organic solvent is removed; and the particles are extracted and dried. Thus, polyester resins used for such a chemical method need to be soluble in organic solvents.
Namely, polyester resins suitable for use in both pulverization and chemical methods need to exhibit all the necessary properties, that is, solubility in solvents, pulverization into fine particles, and storage stability of a toner to be produced.
As an example of such polyester resins, Patent Literature 1 describes a method for enhancing the storage stability of a toner by copolymerizing polyethylene terephthalate with erythritane so as to raise the glass transition temperature. Also, Patent Literature 2 describes a polyester resin with enhanced resin pulverization properties obtained by copolymerizing trifunctional monomers.